In the episode "Eight 'O Clock at the Oasis", Lorelai and Rory go to their neighbor's house to water his plants. In the scene where the girls open his cabinets there is a large selection of board games, including Sorry, Operation, Hungry Hippos, ect. The slip-up occurs when Rory remarks, "He's got Monopoly from every country in the world!" If you look closely you will see that he doesn't even have one Monopoly, let alone one from every country. Besides, there are no versions of Monopoly from other countries. Also, how would Rory even know that, she barely had a glance at the games in the cabinet.

One might say that this wasn't a slip up and that Rory's remark was simply a joke, as the Gilmore Girls are known for their sarcasm and humor, but the way Rory said it was more excitedly rather than sarcastically. So I suppose it was a slip up.